A well-known quenching method for metal is to quench a workpiece preheated at an initial quenching temperature to room temperature before annealing the workpiece. Also, there is an austempering method, a martempering method and a marquenching method, referred to as “isothermal holding heat treatment”, to hold a given intermediate temperature based on an S-curve (T, T, T curve) for a certain period.
Another well-known method is isothermal holding heat treatment with a salt bath disclosed in “Heat Treatment with Salt Bath” published by The Japan Society for Heat Treatment, Aug. 30, 2000, p. 144-147. The salt bath is prepared at 150 to 550 degrees Celsius as a low temperature, 570 to 950 degrees Celsius as a intermediate temperature, and 1000 to 1300 degrees Celsius as a high temperature. Materials for the salt bath are KNO2, a mixture of KNO3 and NaNO2, a mixture of BaCl2 and KCl or CaCl2, or a mixture of NaCl and LiCl or KCl, depending on zones of temperature.
Some solutions have been proposed so as to solve a difficult problem of quenching the workpiece removed from a preheating furnace in the process of the heat treatment with the salt bath. The proposed solutions are shown in “Salt Bath” published by The Japan Society for Heat Treatment, Aug. 30, 2000, p. 769-773. For example, one of the proposed solutions includes a step of providing a salt bath in a vacuum furnace to be heated, a step of removing a workpiece with salt on the face of the workpiece, a step of sequentially moving the workpiece with an anticorrosive coating to the salt bath at a lower temperature, and a step of holding a given intermediate temperature of the salt bath. The workpiece is moved between salt bathes with chain block and the like. Because of the heated workpiece and the heated salt bath at high temperature, skill and caution are needed to perform this solution.
Another solution, referred to as “continuous heat treatment”, includes a step of providing a salt bath for an austempering operation in the lower part of a preheating furnace, a step of moving a workpiece into a salt bath from the preheating furnace, and a step of sequentially moving the workpiece with a belt conveyor. The solution has a problem in that there are many restrictions for the workpiece under the austempering operation.
Still another solution, referred to as “versatile heat treatment”, includes a step of austempering a workpiece with a preheating furnace connected to a quenching salt bath by use of a fully automated operation of tray batch automating transportation. This solution still requires a salt bath and an extensive operation thereof. Those solutions described above requires the steps of soaking a workpiece in a salt bath and removing the workpiece therefrom, and a step of quenching the workpiece or holding the workpiece at an isothermal holding temperature. Therefore, means for soaking and removing the workpiece in those solutions creates a restriction on the heat treatment in itself. For example, in the temperature programmed austempering method, after a workpiece is quenched to 200 degrees Celsius, the workpiece is heated to 250 degrees Celsius, the workpiece is maintained at the isothermal holding temperature, and then the workpiece is quenched again. A plurality of salt bathes are required, and means for moving the workpiece to the salt bathes for these complicated steps, increases the scale of a plant and the cost of the products.
Because of environmental pollution caused by a use of a salt bath, a quenching method without a salt bath is proposed. The method which omits the salt bath includes steps quenching and cooling a workpiece with an inert gas, such as N2 or Ar. According to the method of Japanese Patent Laid Open Publication (Kokai) No. 5-66090 includes a step of accommodating a workpiece in a pressure tight vacuum furnace to heat the workpiece at 1000 to 1200 degrees Celsius, a step for intake of inert gas at 5 degrees Bar so as to quench the workpiece, and a step of circulating the gas with a turbo blower to quench the workpiece relatively quickly. The vacuum furnace includes means, such as a heater, for performing operations on the workpiece for a sequence of heating treatments such as a cooling treatment, a primary preheating treatment, a secondary preheating treatment, a preheating treatment for quenching, and an operation to hold the preheating temperature. After the sequence of heating treatments, N2 gas is pulled into the furnace to quench the workpiece, completing a quenching treatment. This type of furnace is referred to as “a blast furnace”.
The blast furnace is able to quench the preheated workpiece to room temperature for the quenching treatment. However, the blast furnace is not able to hold the isothermal holding temperature at an intermediate temperature for the heating treatment required by the austempering method, the martempering method and the marquenching method.
A way for the austempering method to hold an isothermal holding temperature in the blast furnace is by setting a constant temperature for a certain period as a target temperature controlled by a control unit. The control unit turns on the heater when the temperature falls below the target temperature, and turns off the heater when the temperature exceeds the target temperature.
The gas circulated by the blower is constantly cooled by a cooling system. Therefore, the temperature in the blast furnace varies drastically from 100 degrees Celsius below the target temperature to 50-100 degrees Celsius above the target temperature. Because of the drastic variation of temperature, the blast furnace is unable to hold an isothermal holding temperature. Because the inert gas introduced into the furnace is at room temperature, a problem of partial undercooling arises. It is difficult to heat the inert gas to solve the problem.